Thermocouple having tungsten-rhenium alloy leg wires



March 24, 1970 E. D. ZYSK ET AL 3,502,510

THERMOCOUPLE HAVING TUNGSTEN-RHENIUM ALLOY LEG WIRES Filed Oct. 6. 1966Qe-QaZTUNGsTEN AND H 2-4ZRHENIUM 74-8OZTUNGSTEN AND 2 2026ZRHENIUMINVENTORS EDWARD D. ZYSK BYEUGENE E. OSOVITZ Patented Mar. 24, 19703,502,510 THERMOCOUPLE HAVING TUNGSTEN-RHENIUM ALLOY LEG WIRES Edward D.Zysk, Livingston, and Eugene E. Osovitz, Englishtown, N.J., assiguors toEngelhard Industries, Inc.,

Newark, N .J a corporation of Delaware Filed Oct. 6, 1966, Ser. No.584,822 Int. Cl. H01v 1/14 US. Cl. 136-227 Claims ABSTRACT OF THEDISCLOSURE A thermocouple having a leg and lead wire combinationcomprising a positive leg wire of 9698% tungsten and 24% rhenium joinedto a negative leg wire of 74- 80% tungsten and 2026% rhenium, a leadwire connected to the positive leg composed of 95 %97.5 nickel, 1.5%2.2%chromium, 0.7%l.1% aluminum, 0.3%1.0% silicon and 0%1.2% tungsten, and alead wire connected to the negative leg composed of 95-97% nickel and3%5% tungsten, said percentages of ingredients being by weight.

The present invention relates to thermocouples and particularly to legand lead wire combinations for high temperature thermocouples.

Thermocouples are formed by connecting two wires of dissimilar metalshaving a difference in electric potential which varies as thetemperature at the junction of the wires, referred to as the hotjunction, varies. The difference in potential is measured with apotentiometer, or other instrument for measuring (electromotive force),connected across the wires at their free ends.

The thermocouple must, of course, be made of metals Which withstandtemperatures in the range in which the thermocouple is intended to beused. Beyond that, metals are selected which will produce the largestand most uniform variation in output per degree of temperature changewithin the range in which it is desired to have the thermocouple mostaccurate.

In many cases the metals which are best adapted for accurate hightemperature thermocouples are the more expensive metals. Consequently,it is desirable to be able to have the expensive thermocouple leg wiresas short as possible and use less expensive metal lead wires to connectthe legs to the instrument for measuring potential difference. Butfinding lead wires to match the leg wires is a problem.

For a match the lead Wires must not distort the output of the leg wiresto such extent that it interferes with the accuracy of the thermocouple.,Since the lead wires are of diiferent metals from the leg wires theyform couples at the connections which produce responses dependent uponthe temperature at the connections. These connections are referred to asthe leg wirelead wire junctions and the temperature at these junctionsis determined by the heat conducted along the leg wires from the hotjunction and by the surrounding temperature. The temperature at the legwire-lead wire junctions is therefore lower than but proportional to thetemperature at the hot junction and the amount of difference dependsupon the length of the legs and the extent to which the leg wire-leadwire junctions are insulated from the surroundings.

For a suitable match the output of a couple formed by the leg wiresshould be substantially the same as the E.M.F. output of a couple formedby the lead wires at the temperatures expected at the leg wire-lead wirejunctions when the temperature at the hot junction is in the range inwhich the thermocouple is intended to be used. In a sense theconsideration of the lead wires as a couple and the leg wires as acouple at temperatures expected at the leg wire-lead wire junctions ishypothetical, since the couples which exist in the operatingthermocouple are formed by the junctions of the respective leg and leadwires. However, in testing for a suitable match these hypotheticalcouples are actually formed and tested within the contemplatedtemperature range for if these hypothetical couples have similarresponse then the respective leg and lead combinations will have similarresponses and thus be suitably matched.

The present invention is a high temperature thermocouple having legwires of tungsten-rhenium alloys and matching lead wires. Rhenium is anexpensive metal and the present invention has the advantage of providinga thermocouple which has high response to temperature changes within arange of high temperature due to the rhenium alloyed with tungsten andyet the cost is kept down without sacrificing accuracy by providingmatching lead wires of less expensive metals.

In addition the lead wires are easier to bend and shape around cornersand obstructions than the tungsten-rhenium allow leg wires and thusfacilitate the connection to a measuring instrument.

The objects, advantages and details of the thermocouple of the presentinvention will be more apparent from the following description andaccompanying schematic diagram of a thermocouple in accordance with thisinvention.

As seen in the diagram a thermocouple in accordance with the presentinvention has a hot junction, marked A, at which a thermoelectric legwire 11 is joined to another thermoelectric leg wire 12 of a difierentmetal by welding or other appropriate means to make a good connection.The thermoelectric response of the leg wires 11 and 12 to heat at thehot junction A is measured by a potentiometer 13 or other instrument formeasuring the diflerence in electric potential. As mentioned above theleg wires 11 and 12 are made as short as possible and are connected tothe potentiometer 13 by lead wires 14 and 15 of less expensive metal.

As shown, the lead wires 14 and 15 are connected to the leg wires 11 and12 respectively at B and C which are the leg wire-lead wire junctions.Since the leg and lead wires are of difierent metals, they form coupleswhich have outputs determined by the temperature at the junctions B andC. In order for the lead wires to match the leg wires the output of thelead and leg wire couples at the junctions B and C should be closelyequal over the temperature range expected at these junctions.

In accordance with the present invention the positive thermoelectric legwire is an alloy of 96-98% tungsten and 24% rhenium. The other ornegative leg wire is an alloy of 74-80% tungsten and 2026% rhenium. Allpercentages of ingredients of the alloys described herein are to beunderstood as being percentages by weight.

Preferred alloys used for the leg wires are 97% tungsten and 3% rheniumfor the positive leg and 75% tungsten and 25% rhenium for the negativeleg but the lead wires described in detail below are adapted for usewith leg wires whose proportions of tungsten and rhenium may be variedwithin the ranges set out in the previous paragraph.

In addition, in practice, minute traces of materials such as thoriumoxide, potassium, aluminum or silicon are sometimes added to the alloysof the leg wires to control grain growth and thereby increase the usefullife of the wires at the high temperatures to which they are subjected.The inclusion of these materials does not alter the thermoelectricproperties of the above described leg wire alloys sufficiently to reducethe precision at which the tungsten-rhenium couples are used and it isto be understood that the above alloys with grain growth controllingmaterials added are intended to be included in the scope of theinvention.

Previous thermocouples having a 100% tungsten leg versus a leg of 74% or75% tungsten and 25% or 26% rhenium tend to fail rapidly in service dueto recrystallization and grain growth of the tungsten leg. A couplehaving a 95% tungsten-% rhenium leg versus a 75% tungsten-% rhenium legwas tested and found to provide successful high temperature operationwithout a resulting embrittlement at low temperatures than a tungstenversus tungsten-25% rhenium couple. However, the 100% tungsten versus75% tungsten-25% rheniurn couple produces higher E.M.F. response attemperatures around 2000 C. and above.

The thermocouple in accordance with the present invention having a legof 96-98% tungsten-24% rhenium--and preferably 97% tungsten-3%rheniumversus a 75% tungsten-25% rhenium leg has advantages over coupleswith either a 100% tungsten leg or a tungsten-5% rhenium leg versus aleg of 75% tungsten- 25% rhenium. The 95% tungsten-3% rhenium leg.particularly when doped with trace amounts of grain growth controllersas described above, has a much longer useful life than the tungsten leg,and frequently in tests exhibited greater high temperature mechanicalstability than the 95 tungsten-5% rhenium leg.

The E.M.F. response of a couple of this invention having a 97%tungsten3% rhenium leg versus a 75% tungsten-25% rhenium leg is higherthan that of a couple with a 100% tungsten leg versus a 75% tungsten-25% rhenium leg over the range tested. its E.M.F. is notably higher thanthe couple with the 95% tungsten- 5% rhenium leg at all workingtemperatures above 1000 C. Comparative EMF. measurements are shown inthe following Table 1.

TABLE l.-EMF IN MILLIVOLTS Hot junction W vs. 97% W, 3% 95% W, 5% 95% W,5% Temperature, 74% W, Re vs. 75% Re vs. 75% Re vs. 74% Of 26% Re W, 25%Re W, 25% Re W, 26% Re Reference junction at measuring instrumentmaintained at 0 C.

The couple having a leg wire of 97% tungsten-3% rhenium versus a legwire of 75% tungsten-25% rheniurn produces high and uniform variationsin per degree of temperature change within a range from 0 to about 2700C. at the hot junction A. This couple is considered best of the couplesof the above Table I for use at the higher temperatures in this range.At the higher temperatures the couples should be protected fromoxidation. Suitable protection against oxidation is provided bysurrounding the couple with an atmosphere of hydrogen, helium, argon,and nitrogen-hydrogen mixtures. A good vacuum is also suitable.

In accordance with the present invention the negative lead wire for thenegative leg wire of 74-80% tungsten- 2026% rhenium is an alloy leadwire composed of 95%97% Ni and 3%5% W. One suitable negative lead wireused in practice is an alloy wire composed of 96% Ni and 4% W. The leadwire for the positive leg wire of the 9698% tungsten-24% rhenium leg isan alloy wire of 95%-97.5% Ni, 1.5%-2.2% Cr, 0.7%- 1.l% A1, 0.3%1.0% Siand 0%1.2% W and a particular positive leg alloy lead wire used inpractice contains 96.5% Ni, 2% Cr, 1% Al, and 0.5% Si.

In the usual arrangement the leg wire-lead wire junctions B and C areclose together and are the same distance from the hot junction A. Thetemperature at both junctions B and C will therefore be substantiallythe same. As previously discussed the difference between thetemperatures at the hot junction A and the leg wire-lead wire junctionsB and C is determined by the distance of the junctions B and C from thehot junction A and the extent to which the junctions B and C areinsulated from the surroundings. The range of temperatures at thejunctions B and C when the hot junction A is within the range oftemperatures which the thermocouple is adapted to measure can thereforebe adjusted and fixed by varying the length of the legs and varying theinsulation of the leg wire-lead wire junctions. It will be appreciatedthat extensive experimentation and testing are required to find leadwire combinations which closely match the leg wire combinations over anyrange of temperatures. When lead wire combinations are found whichclosely match the leg wire combination over a range of temperaturesapproximately equal in extent to the range of temperatures to which theleg wire-lead Wire junctions will be subjected, then it may be desirableto shift the range effective at the leg wire-lead wire junctions byadjustment as above in order to achieve the best match.

With the hot junction A subjected to temperatures in a range up to about2700 C. which the thermocouple of this invention is intended to measure,the leg wire-lead wire junctions B and C are therefore spaced from thehot junction A and insulated so as to have a corresponding lowertemperature within the range of 0 C.400 C. and preferably in the rangeof 0 C.350 C.

As used herein a suitable match between the leg and lead wires is one inwhich the variation in output of the legs per degree of temperaturechange does not differ more than about 10% from the variation in outputof the lead wires resulting from the same change of temperature from atemperature at which the E.M.F. outputs are the same.

In Table II below the close match of lead and leg wires of the presentinvention are illustrated by comparing the potential difference oroutput of the leg wire couple with the potential difference of output ofthe lead wire couple.

TABLE 11.E.U.F. IN MILLIVOLTS Leg Wires Lead Wires 97% W, 3% Be 96.5%Ni, 2% VS. 75% W, Cr, 1% A1. 0.5% 25% Re Si vs. 96% Ni, Temperature, C.4% W Difference It will be appreciated that the above table is illustra-Apart from the close match of the lead wires with their respective W-Rethermocouple legs, they are capable of withstanding, withoutdeterioration, temperatures from 300 (3.400 C. in an oxidizingatmosphere such as air and they also exhibit a very favorable welding totheir respective W-Re thermocouple legs.

Variations of the thermocouple and lead wire compositions arecontemplated within the scope of the appended claims. I

What is claimed is:

1. A thermocouple having a leg and lead wire combination comprising apositive leg wire of 9698% tungsten and 24% rhenium joined to a negativeleg wire of 7480% tungsten and 20-26% rhenium, a lead wire connected tothe positive leg composed of 95%97.5% nickel, 1.5%2.2% chromium,0.7%1.1% aluminum, 0.3%1.0% silicon and 0%1.2% tungsten, and a lead wireconnected to the negative leg composed of 95%- 97% nickel and 3%5%tungsten, said percentages of ingredients being by weight.

2. A thermocouple leg and lead wire combination as set forth in claim 1in which the positive leg wire is about 97% tungsten and about 3%rhenium.

3. A thermocouple leg and lead wire combination as set forth in claim 1in which the negative leg wire is about 75% tungsten and about 25%rhenium.

4. A thermocouple leg and lead wire combination as set forth in claim 1in which the lead wire connected to the negative leg is 96% nickel and4% tungsten.

5. A thermocouple leg and lead wire combination as set forth in claim 1in which the lead wire connected to the positive leg is 96.5% nickel, 2%chromium, 1% aluminum, and 0.5% silicon.

References Cited UNITED STATES PATENTS 3,296,035 1/1967 Zysk l36227FOREIGN PATENTS 1,021,134 2/1966 Great Britain.

WINSTON A. DOUGLAS, Primary Examiner M. I. ANDREWS, Assistant ExaminerUS. Cl. X.R. 136236

